1. Field of the Invention
This invention relates to metal air cells, and particularly to a metal air cell system having a novel configuration promoting efficient cell discharge and simplified oxidant management.
2. Description of the Prior Art
Electrochemical power sources are devices through which electric energy can be produced by means of electrochemical reactions. These devices include metal air electrochemical cells such as zinc air and aluminum air batteries. Certain metal electrochemical cells employ an anode comprised of metal particles that are fed into the cell and consumed during discharge. Such electrochemical cells are often called refuelable batteries. Zinc air refuelable battery cells include an anode, a cathode, and an electrolyte. The anode is generally formed of zinc particles immersed in electrolyte. The cathode generally comprises a semipermeable membrane and a catalyzed layer for electrochemical reaction. The electrolyte is usually a caustic liquid that is ionic conducting but not electrically conducting.
Metal air electrochemical cells have numerous advantages over traditional hydrogen-based fuel cells. Metal air electrochemical cells have high energy density (W*hr/Liter), high specific energy (W*hr/kg), and run at ambient temperature. Further, the supply of energy provided from metal air electrochemical cells is virtually inexhaustible because the fuel, such as zinc, is plentiful and can exist either as the metal or its oxide. The fuel may be solid state, therefore, safe and easy to handle and store. In contrast to a hydrogen-oxygen fuel cell, which uses methane, natural gas, or liquefied natural gas to provide as source of hydrogen, and emit polluting gases, the metal air electrochemical cells results in zero emission.
The metal air electrochemical cells operate at ambient temperature, whereas hydrogen-oxygen fuel cells typically operate at temperatures in the range of 150° C. to 1000° C. Metal air electrochemical cells are capable of delivering higher output voltages (1.5-3 Volts) than conventional fuel cells (<0.8V). Due to these advantages, metal air electrochemical cells can be used as power sources of all kind of applications, like stationary or mobile power plant, electric vehicle or portable electronic device, etc.
One of the principle obstacles of metal air electrochemical cells is the prevention of leakage of the electrolyte, typically a liquid electrolyte. For example, during refueling, the electrolyte can leak out and contaminate the user. Another obstacle relates to cell failure due to anode degradation. Where refuelability is provided, the anode and the cathode should have clearance between them. However, this clearance will increase uneven discharging at two of the major anode surfaces. Further, the clearance increases the internal resistance between the anode and cathode. The uneven discharge will reduce the life of the anode, and the power output and the life the metal air cell.
Another obstacle of metal air electrochemical cells relates to both oxygen and thermal management. Regarding thermal management, typical systems involve electrolyte circulation, which generally require multiple fluid transport components such as piping structures, pumps, and radiators. These fluid transport components minimize the overall system energy density and specific energy. Typical zinc air systems provide the same airflow for chemical reaction and also to remove heat. Even where electrolyte is circulated, is generally heat exchanged through air.
A further obstacle of metal air electrochemical cells is the inherent volume expansion of the metal, wherein the electrode shape may vary. Electrode shape change generally involves migration of zinc from the certain regions of the electrode to other reasons, and occurs, in part, as the active electrode material dissolves away during battery discharge. Swelling and deformity of zinc electrodes also occur due to the differences in volume of metallic zinc and its oxidation products zinc oxide and zinc hydroxide. Electrode shape distorts as the zinc is redeposited in a dense solid layer, thereby minimizing available active electrode material and preventing electrolyte access to the electrode interior.
Yet another obstacle relates to refueling of metal air cells. If the clearance between the anode and cathode is not large enough to accommodate the anode expansion, the cathode may be damaged and hence render refueling difficult or impossible. The distance between anode and cathode should be constant. If the distance between the anode and cathode is not constant, the discharging between the anode and cathode will be uneven. This uneven discharging will cause the anode to bend or deform. This bend on the anode is caused by the volume change due to the metal oxidation. When the anode is bent, the anode area which closer to the cathode discharges faster than the rest of the anode. This will increase the deformation. Therefore, the uneven discharging is magnified, and the problem continues until the bending causes cell failure, for example by shorting with the anode. Also, the uneven discharging will reduce the power output of the cell. If the cell is discharged at very high power, the regions of the anode closer to cathode will be passivated and lose functionality.
In order to refuel, the anode and cathode should have certain distance between them to provide the clearance for the refueling action. Conventionally, this clearance is filled with electrolyte and separator. However, this clearance will increase the cell internal resistance. This internal resistance will generate heat during use, which may cause various detriments. The heat consumes power from the cell, will dry out the electrolyte quickly, and speeds up the deterioration of the fuel cell. In order to reduce the internal resistance, the distance between the anode and cathode should be small and even. Nonetheless, this conventionally sacrifices durability. During the refueling process, if the distance between anode and cathode is not sufficient, the anode may scrape the cathode surface. Excess clearance, while reducing the likelihood of cathode damage during the refueling, increases the internal resistance. Therefore, conventionally provision of sufficient clearance between the anode and cathode results in increased internal resistance between them.
Therefore, a need remains in the art for a metal air cell that is refuelable, does not leak, minimizes anode degradation due to clearance between the anode and the cathode, and includes an efficient system for oxygen and thermal management.